1. Field of the Invention
The present invention relates to an optical module and a sealing method therefor.
2. Description of the Related Art
In recent years, the development of an optical access network has been actively pursued. For realization of an optical access network, it is greatly important to reduce the cost of an optical device and/or an optical module. Also in sealing an optical element for performing opto-electric conversion or electro-optic conversion in an optical module, further cost reduction is required, and a method of easily sealing the optical element with a sufficient reliability is demanded.
In many optical modules at present, a metal package or ceramic package is used to ensure reliability and the package is hermetically sealed by welding or soldering, resulting in a very high cost of optical module. For cost reduction of an optical module, it is now important to simplify the sealing method for an optical element. As an example of simplification of the sealing method for an optical element, there has been proposed a method of applying a resin to the entire surface of a substrate on which an optical element is mounted and next curing the resin (Mitsuo Fukuda et al., "Plastic Packaging of Semiconductor Laser Diode", Electronic Components and Conference, 1996, pp1101-1108).
In the case of applying a resin to the entire surface of a substrate on which an optical element is mounted and next curing the resin to seal the optical element as described in the above literature, there is a possibility that separation or cracking of the resin may occur because of a large difference in coefficient of thermal expansion between the substrate and the resin, or the substrate may be broken by a residual stress in the resin. The larger the thickness of the resin coating covering the optical element and the wider the range of spread of the resin coating, the larger the residual stress in the resin. Accordingly, the residual stress can be relaxed by thinly applying the resin to a minute area surrounding the optical element. However, the resin is generally in the form of gel, so that when dropped onto the substrate, it spreads widely on the substrate and it is difficult to apply the resin to only the minute area surrounding the optical element.